The present invention relates to a process for gas-phase oxidation, in which a gaseous stream comprising an aromatic hydrocarbon and molecular oxygen is passed through two or more catalyst zones. Furthermore, the present invention relates to a catalyst system for gas-phase reaction using a preliminary zone.
Many carboxylic acids and/or carboxylic anhydrides are prepared industrially by catalytic gas-phase oxidation of aromatic hydrocarbons such as benzene, the xylenes, naphthalene, toluene or durene in fixed-bed reactors. In this way, it is possible to obtain, for example, benzoic acid, maleic anhydride, phthalic anhydride, isophthalic acid, terephthalic acid or pyromellitic anhydride. In general, a mixture of an oxygen-comprising gas and the starting material to be oxidized is passed through tubes in which a bed of a catalyst is present. To regulate the temperature, the tubes are surrounded by a heat transfer medium, for example a salt melt.
Although the excess heat of reaction is removed by the heat transfer medium, local temperature maxima (hot spots) in which the temperature is higher than in the remainder of the catalyst bed can be formed in the catalyst bed. These hot spots lead to secondary reactions, e.g. total combustion of the starting material, or to formation of undesirable by-products which cannot be separated, or separated only with great difficulty, from the reaction product.
In addition, the catalyst can be irreversibly damaged above a particular hot spot temperature. For this reason, the loading of the gaseous stream with the hydrocarbon to be oxidized has to be kept very low at the beginning when starting up the process and can be increased only slowly. The final production state is often reached only after a few weeks.
In recent years, multizone catalyst systems have been used for the oxidation of aromatic hydrocarbons (for example DE-A 40 13 051, DE-A 198 23 262, EP-A 1 063 222, WO 2005/115616, EP-A 1 084 115, DE-A 103 23 818, DE-A 103 23 461, DE-A 103 23 817). The objective is to match the activity of the individual catalyst zones to the reaction profile along the axis of the reactor. This makes it possible to achieve a high yield of desired product and at the same time a very low yield of the undesirable intermediates or by-products. The catalysts of the first zone closest to the reactor inlet usually have the lowest activity since the highest starting material concentration and thus the greatest reaction rate occur in the region close to the reactor inlet. The heat liberated in the chemical reaction heats the reaction gas up to the point at which the energy produced by the reaction is the same as the energy given off to the coolant. An excessively high activity in the first catalyst zone would lead to an uncontrolled increase in the hot spot temperature, which can usually lead to a reduction in selectivity or even to the reaction becoming uncontrollable.
A further aspect which has to be taken into account in configuring the activity of the individual catalyst zones is the position of the hot spot in the first catalyst zone. Since the catalyst activity decreases with increasing time of operation, a higher proportion of unreacted hydrocarbons or partially oxidized intermediates gets into regions of the catalyst bed located further downstream. The reaction thus moves further toward the reactor outlet and the position of the hot spot shifts ever further in the direction of the reactor outlet. This can even lead to the hot spot migrating from the first catalyst zone into the second or a subsequent catalyst zone. This migration of the hot spot results in a significant decrease in the yield of desired product.
The deactivation of the catalyst can be countered to a limited extent by increasing the temperature of the heat transfer medium. The increase in the temperature of the heat transfer medium and/or the movement of the hot spot lead, in the case of multizone catalyst systems, to the temperature at which the gas mixture enters a subsequent catalyst zone increasing. Since downstream catalyst zones are generally more active but less selective, undesirable overoxidation and other secondary reactions increase. These two effects result in the product yield or selectivity decreasing with increasing time of operation. In such a case, a complete replacement of the catalyst can be more economically advantageous than continuation of operation.
A disadvantage of the activity-structured catalyst systems reported in the prior art is that, despite the use of such structured catalyst systems, the life of the catalysts is not satisfactory, in particular in respect of the continual movement of the hot spot in the direction of the gas stream. A positioning of the hot spot in a more active catalyst zone further toward the gas outlet also restricts the opportunity of making a fine adjustment in the selectivity of the catalyst to avoid undesirable by-products.
WO 2006/92305, WO 2006/92304 and the European patent application number 06112510.0 solve this problem by the use of an active catalyst zone which is located upstream of the usual catalyst zones toward the gas outlet. This active preliminary zone effects more rapid heating of the reactor gases and thus an earlier start of the chemical reaction, so that the hot spot forms further toward the gas outlet compared to the systems of the prior art.
However, a disadvantage of a more active preliminary zone is that a further suspension of active compositions for the preliminary zone has to be prepared in addition to the suspensions of active compositions for the subsequent three to four catalyst zones. Furthermore, a very active first zone at the reactor inlet represents an increased safety risk since the reaction can become uncontrollable because of the high proportion of starting material at the reactor inlet.
A further disadvantage of the prior art is that the gas temperature at the reactor inlet is far below the salt bath temperature. In this region, the salt bath serves not to remove excess heat but instead to heat the reaction gas. An unfilled, empty tube, a tube filled with inert spherical material or a tube filled with catalyst is usually used for heating the reaction gas. The disadvantage of inert spherical material is the high pressure drop over the reactor.